Touch cell structure of a touch panel and the touch panel using the same

ABSTRACT

Provided is a touch cell structure for a touch panel in which a touch cell is configured into a new pad to gate mode in order to solve a problem of a conventional capacitive-type touch input device. The touch cell structure includes: a conductive pad that forms an electrostatic capacitance with respect to a touch unit when a finger of a human body or the touch unit having an electrical characteristic similar to the finger approaches the conductive pad within a predetermined distance; and a three-terminal type switching device whose gate terminal is connected with the conductive pad and whose output signal is changed in correspondence to a change in electric potential of the gate terminal of the three-terminal type switching device by the electrostatic capacitance between the touch unit and the conductive pad. Since the potential of the gate terminal of the switching device is determined by an electrostatic capacitance formed in the conductive pad, a difference of the output signal output from the switching device becomes large depending on whether a touch input occurs or not.

TECHNICAL FIELD

The invention relates to a touch cell structure of a touch panel and thetouch panel using the same, and more particularly, to a touch cellstructure having a very high detection sensitivity and accuracy fortouch inputs as a specialized cell configuration, and employing a newpad to gate mode that can detect touch inputs in a digital mode andrecognize multi-touch inputs and a touch panel using the same.

BACKGROUND ART

Usually, touch input devices are input devices which are respectivelyattached onto display devices such as LCDs (Liquid Crystal Displays),PDPs (Plasma Display Panels), OLED (Organic Light Emitting Diode)displays, and AMOLED (Active Matrix Organic Light Emitting Diode)displays or which are respectively built in the display devices andrecognize touch input signals when an object such as a finger or pencontacts a respective screen on touch panels.

Recent years, the touch input devices are respectively mounted on mobiledevices such as mobile phones, PDAs (Personal Digital Assistants) orPMPs (Portable Multimedia Players). Besides, touch input devices arebeing used over all industries such as navigation terminals, netbookcomputers, notebook computers, DIDs (Digital Information Devices),desktop computers that use touch input supporting operating systems,IPTVs (Internet Protocol TVs), the most advanced fighter aircrafts,tanks, and armored vehicles.

Various types of conventional touch input devices are disclosed, butresistive-type touch input devices having simple manufacturing processesand inexpensive manufacturing costs have been used most widely. Theresistive-type touch input devices or touch panels, however, have lowtransmittance and undergo the pressure applied to respective substrates,to thereby cause problems that inevitable loss of durability occurs overlapse of use time, it is difficult to accurately perceive touch points,and detection errors occur frequently due to surrounding environmentssuch as temperature and the noise.

Capacitive-type or electrostatic capacitive-type touch input devicesthat are developed as an alternative to the resistive-type touch inputdevices detect touch inputs in a non-contact mode and have a solution tovarious problems of the resistive-type touch input devices.

FIG. 1 shows the structure of a conventional electrostaticcapacitive-type touch panel. Referring to FIG. 1, the conventionalcapacitive-type touch panel includes transparent conductive films thatare formed on the top and bottom surfaces of a transparent substrate 10made of film, plastic or glass. Metal terminals 12 for applying voltageare formed at each of four corners of the transparent substrate. Thetransparent conductive film is formed of transparent metal such as ITO(Indium Tin Oxide) or ATO (Antimony Tin Oxide). The metal terminals 12respectively formed at the four corners of the transparent conductivefilm are formed by printing low resistivity conductive metal such assilver (Ag). Resistor network is formed around the metal terminals 12.The resistor network is formed in a linearity pattern in order totransmit a control signal equally on the entire surface of thetransparent conductive film. A protective film is coated on top of thetransparent conductive film including the metal terminals 12.

The capacitive-type touch panels operate as follows. A high-frequencyalternating-current (AC) voltage applied to the metal terminals 12, isspread to the whole surface of the transparent substrate 10. Here, if afinger 16 for a conductive material touch means) lightly touches the topsurface of a transparent conductive film of a transparent substrate 10,an amount of electric current is absorbed into the human body andchanges in the electric current are detected by a built-in electriccurrent sensor of a controller 14, to thus calculate the amount ofelectric current at the four metal terminals 12, respectively, and tothereby recognize a touch point.

The capacitive-type touch panel employs a soft touch mode to thus have along life, uses only a sheet of the transparent substrate 10, to thushave a high light transmittance, and makes a special metal coatingtreatment on a contact surface thereof, to thus have an advantage ofrobustness. In particular, the capacitive-type touch panel has a narrowwidth of a non-active area which makes it impossible to detect touchinputs at the panel edge portions, to thus have an advantage of enablinga mechanical instrument to be made in a slim form at the time of beingcoupled with a display device.

However, the electrostatic capacitive-type touch panel needs anexpensive detector in order to detect a magnitude of minute electriccurrent, and further needs an analog-to-digital (ADC) converter forconverting detected analog electric current to digital electric current,to accordingly cause an inevitable price increase. In addition, theremay raise a problem that a response time is prolonged due to the timeconsumed for converting analog signals to digital signals. Above all,since a difference in magnitude between an electric current detectedwhen a touch input occurs and a usual electric current measured beforethe touch input is very small, there may cause bad detection sensitivityand high noise sensitivity. For example, assuming that a magnitude ofelectric current that is leaked from one of the metal terminals 12 whenno touch input occurs is 1 μA and a magnitude of electric current thatis leaked from the same one metal terminal 12 when a touch input occursis 2 μA, detection of the difference between the minute electriccurrents by using a circuitry means may cause degradation of accuracyand signal recognition errors due to noise.

DISCLOSURE Technical Problem

In order to solve the above-mentioned problems of requiring for acomplex configuration to detect minute signal changes due to a touchinput in a conventional electrostatic capacitive touch input device, itis an object of the present invention to provide a touch cell structureenlarging a difference between detection signals depending upon touchinputs and accordingly having a very high detection sensitivity andaccuracy for touch inputs as a unit touch cell constituting a touchinput device has a specialized circuitry configuration, and employing anew mode that can detect touch inputs in a digital mode without using anexpensive component such as an analog-to-digital (ADC) converter to thusgreatly reduce a response time, remove a misrecognition due to noise andrecognize multi-touch inputs and a touch panel using the same.

Technical Solution

To attain the above object of the present invention, according to anaspect of the present invention, there is provided a touch cellstructure constituting a unit touch cell 60 in a touch panel, the touchcell structure comprising:

a conductive pad 50 that forms an electrostatic capacitance with respectto a touch unit when a finger 25 of a human body or the touch unithaving an electrical characteristic mar to the finger approaches theconductive pad 50 within a predetermined distance “d”; and

a three-terminal type switching device 40 whose gate terminal isconnected with the conductive pad 50 and whose output signal is changedin correspondence to a change in electric potential of the gate terminalof the three-terminal type switching device 40 by the electrostaticcapacitance between the touch unit and the conductive pad 50.

Preferably but not necessarily, the switching device 40 comprises:

a first three-terminal type switching unit 42 whose output signal isconnected with the conductive pad 50, and that is turned on/offaccording to a control signal applied to a gate terminal of the firstthree-terminal type switching unit 42, to thereby supply a chargingsignal to the conductive pad 50; and

a second three-terminal type switching unit 44 whose gate terminal isconnected with the conductive pad 50 and whose output signal is changedin correspondence to a change in electric potential of the gate terminalof the three-terminal type switching unit 44.

Preferably but not necessarily, the touch cell structure furthercomprises capacitors C1 and C2 that are connected between the controlterminal and the output terminal of the first three-terminal typeswitching unit 42 and the second three-terminal type switching unit 44,respectively.

Preferably but not necessarily, the capacitor C1 connected between thecontrol terminal and output terminal of the first three-terminal typeswitching unit 42 is in the range of 10 fF to 100 uF.

Preferably but not necessarily, the capacitor C1 connected between thecontrol terminal and the output terminal of the first three-terminaltype switching unit 42 is selected to have a smaller value by twice toseveral hundreds of times than a value of a capacitor Ct formed betweenthe touch unit and the touch pad 50.

Preferably but not necessarily, the capacitor C1 connected between thecontrol terminal and the output terminal of the first three-terminaltype switching unit 42 is selected to have a value or more of acapacitor Ct formed between the touch unit and the touch pad 50.

Preferably but not necessarily, the capacitors C1 and C2 are built inthe first three-terminal type switching unit 42 and the secondthree-terminal type switching unit 44, respectively.

Preferably but not necessarily, the capacitors C1 and C2 are provided inthe outside of the first three-terminal type switching unit 42 and thesecond three-terminal type switching unit 44, respectively.

Preferably but not necessarily, the touch cell structure furthercomprises a capacitor C3 connected between the input terminal andcontrol terminal of the second three-terminal type switching unit 44.

Preferably but not necessarily, the touch cell structure furthercomprises an auxiliary capacitor 54 between the conductive pad 50 andthe ground.

Preferably but not necessarily, the switching device 40 is any oneselected from the group consisting of a relay, a MOS (Metal OxideSemiconductor) switch, a BJT (Bipolar Junction Transistor) switch, a FET(Field Effect Transistor) switch, a MOSFET (Metal Oxide SemiconductorField Effect Transistor) switch, an IGBT (Insulated Gate BipolarTransistor) switch, and a TFT (Thin Film Transistor) switch.

To achieve the above object, according to another aspect of the presentinvention, there is provided a touch panel comprising:

a light transmissive substrate 30;

touch cells 60 that are arranged in a matrix form on the lighttransmissive substrate 30, in which each touch cell comprises aconductive pad 50, and a three-terminal type switching device 40 whosegate terminal is connected with the conductive pad 50 and whose outputsignal is changed in correspondence to a change in electric potential ofthe gate terminal of the three-terminal type switching device 40 by anelectrostatic capacitance between a finger 25 of a human body or a touchunit having an electrical characteristic similar to the finger and theconductive pad 50; and

a touch position detector 70 that recognizes a touch input from theoutput of the switching device 40.

Preferably but not necessarily, the switching device 40 comprises:

a first three-terminal type switching unit 42 whose output signal isconnected with the conductive pad 50, and that is turned on/offaccording to a control signal applied to a gate terminal of the firstthree-terminal type switching unit 42, to thereby supply a chargingsignal to the conductive pad 50; and

a second three-terminal type switching unit 44 whose gate terminal isconnected with the conductive pad 50 and whose output signal is changedin correspondence to a change in electric potential of the gate terminalof the three-terminal type switching unit 44.

Preferably but not necessarily, the touch panel further comprisescapacitors C1 and C2 that are connected between the control terminal andthe output terminal of the first three-terminal type switching unit 42and the second three-terminal type switching unit 44, respectively.

Preferably but not necessarily, the capacitor C1 connected between thecontrol terminal and output terminal of the first three-terminal typeswitching unit 42 is in the range of 10 fF to 100 uF.

Preferably but not necessarily, the capacitor C1 connected between thecontrol terminal and the output terminal of the first three-terminaltype switching unit 42 is selected to have a smaller value by twice toseveral hundreds of times than a value of a capacitor Ct formed betweenthe touch unit and the touch pad 50.

Preferably but not necessarily, the capacitor C1 connected between thecontrol terminal and the output terminal of the first three-terminaltype switching unit 42 is selected to have a value or more of acapacitor Ct formed between the touch unit and the touch pad 50.

Preferably but not necessarily, the capacitors C1 and C2 are built inthe first three-terminal type switching unit 42 and the secondthree-terminal type switching unit 44, respectively.

Preferably but not necessarily, the capacitors C1 and C2 are provided inthe outside of the first three-terminal type switching unit 42 and thesecond three-terminal type switching unit 44, respectively.

Preferably but not necessarily, the touch panel further comprises acapacitor C3 connected between the input terminal and control terminalof the second three-terminal type switching unit 44.

Preferably but not necessarily, the touch panel further comprises anauxiliary capacitor 54 between the conductive pad 50 and the ground.

Preferably but not necessarily, the switching device 40 is any oneselected from the group consisting of a relay, a MOS (Metal OxideSemiconductor) switch, a BJT (Bipolar Junction Transistor) switch, a FET(Field Effect Transistor) switch, a MOSFET (Metal Oxide SemiconductorField Effect Transistor) switch, an IGBT (Insulated Gate BipolarTransistor) switch, and a TFT (Thin Film Transistor) switch.

Preferably but not necessarily, the touch position detector 70 appliesan on/off control signal to a control terminal of the first switchingunit 42, applies a position detection signal to an input terminalthereof, and compares a difference of an output signal of the secondswitching unit 44 according to a difference of a kick back dependingupon whether or not a touch input exists, to thus recognize the touchinput.

Preferably but not necessarily, the output signal of the secondswitching unit 44 has tens of times or tens of thousands of times adifference depending upon whether or not a touch input exists.

Preferably but not necessarily, the touch panel further comprises acomparator for comparing the output signal of the second switching unit44 and a reference signal.

Preferably but not necessarily, a sensing cell 61 including athree-terminal sensing switching unit 64, which has the same circuitconfiguration as the second switching unit 44 but to the controlterminal of which the conductive pad 50 is not connected, is furtherprovided at one side of the substrate 30, the touch position detector 70applies a control signal corresponding to a value positioned between asignal applied to the control terminal of a second TFT 44 of the touchcell 60 at the time of no occurrence of touch inputs and a signalapplied to the control terminal of the second TFT 44 of the touch cell60 at the time of occurrence of touch inputs, to the control terminal ofthe sensing switching unit 64, the same signal as the signal applied tothe input terminal of the second TFT 44 is applied to the input terminalof the sensing switching unit 64, and the signal output from the outputterminal of the sensing switching unit 64 is used as a reference signalof the comparator.

Preferably but not necessarily, the touch position detector 70 furthercomprises a memory unit 74 having addresses corresponding to thecoordinates of the touch cell 60, in which if a touch input is detected,coordinate values of the corresponding touch cell 60 are stored in thecorresponding addresses of the memory unit 74.

Advantageous Effects

A touch cell structure and a touch panel using the same according to thepresent invention, includes a conductive pad that forms an electrostaticcapacitance with respect to a finger of a human body or the touch unithaving a conductive characteristic similar to the finger, and athree-terminal type switching device whose gate terminal is connectedwith the conductive pad, that is, the touch cell structure is configuredinto a P2G (Pad to Gate) mode in which the gate terminal of thethree-terminal type switching device is connected with the conductivepad. Accordingly, an electrostatic capacitance Ct formed between thetouch unit and the conductive pad determines an electric potential ofthe gate terminal of the switching device, and the output signal of theswitching device has tens of times or tens of thousands of times adifference depending upon whether or not a touch input exists. Thus,detection sensitivity and detection accuracy with respect to the touchinput are very high, and the touch input can be detected as a high/lawlevel of the output signal, to thus detect the touch input in a digitalmode unlike the conventional touch panel structure using an analog todigital (ADC) converter. In addition, the touch cell structure has avery fast response speed with respect to a touch input signal and haslittle influence due to noise, and thus does not raise a malfunction anda false-recognition of a signal but enables an independent operation ofeach touch cell in an active matrix (AM) mode in which each touch celloperates actively, and a recognition of a multi-touch input that issimultaneously touched at a plurality of touch points. In addition, thetouch cell structure has a specialized cell structure to thus enable agap between cells to become minute and to thereby provide an effect ofpromoting development of an application using touch inputs as well asenabling a touch input support with respect to a variety ofapplications.

DESCRIPTION OF DRAWINGS

The above and other objects and advantages of the invention will becomemore apparent by describing the preferred embodiments with reference tothe accompanying drawings in which:

FIG. 1 is a perspective view showing an example of a conventionalcapacitive-type touch panel;

FIG. 2 is an exploded perspective view showing structure of a touchpanel according to the present invention;

FIG. 3 is a conceptual drawing depicting a method of detecting a touchinput in the present invention;

FIG. 4 is a diagram showing a touch cell structure in accordance with abasic embodiment of the present invention;

FIG. 5 is a conceptual block diagram showing a memory unit according toan embodiment of the present invention;

FIG. 6 shows a configuration according to a preferred embodiment of thepresent invention;

FIG. 7 is a plan view showing structure of a unit touch cell structurein the embodiment of FIG. 6;

FIG. 8 is a cross-sectional view cut along a line I-II of FIG. 7;

FIG. 9 is a waveform diagram illustrating an example of detecting touchinputs according to the embodiment of FIG. 6;

FIG. 10 shows a configuration according to another embodiment of thepresent invention;

FIG. 11 shows a configuration according to a modified embodiment of FIG.10;

FIG. 12 is a cross-sectional view showing an example of capacitorsinternally designed in a TFT;

FIG. 13 shows a configuration of a touch cell illustrating a state wherecapacitors are internally designed in a TFT;

FIG. 14 is a waveform diagram illustrating examples of kick backwaveform depending upon whether or not a touch input exists;

FIG. 15 is a graphical view showing a gate voltage versus output currentcharacteristic of a TFT;

FIG. 16 is a diagram showing an example of detecting a touch input usinga comparator;

FIG. 17 is a waveform diagram illustrating waveform at the time ofdetection using a comparator;

FIG. 18 is a circuitry diagram illustrating a configuration of a sensingcell; and

FIG. 19 is a circuitry diagram showing another example of the sensingcell.

BEST MODE

Hereinbelow, a touch cell structure and a touch panel using the sameaccording to preferred embodiments of the present invention will bedescribed with reference to the accompanying drawings.

First, the present invention relates to a touch cell structure for atouch panel that is added on an upper surface of a display device suchas LCD (Liquid Crystal Display), PDP (Plasma Display Panel), OLED(Organic Light Emitting Diode), and AMOLED (Active Matrix Organic LightEmitting Diode), or that is built in the display device, and a touchpanel using the touch cell structure. The touch cell structure accordingto the present invention means a structure of respective unit touchcells in a cell type touch input device in which an active area thatenables an actual touch input on a touch panel is divided into aplurality of sub-areas, and thus a plurality of touch cells are arrangedin a matrix pattern.

Each unit touch cell structure includes a conductive pad that forms anelectrostatic capacitance relative to a finger or a touch unit having aconductive characteristic similar to that of the finger, and athree-terminal type switching device whose gate terminal is connected tothe conductive pad. Here, the touch cell structure specialized accordingto the present invention is called a P2G (Pad to Gate) type since anelectric potential of the gate terminal of the switching device isdetermined by the electrostatic capacitance of the conductive pad, or iscalled a F2G (Finger to Gate) type since the electric potential of thegate terminal of the switching device is varied by the capacitanceproduced by the finger. It will be easily understood that a P2G or F2Gtype touch cell structure according to the present invention differsfrom the conventional electrostatic capacitive type touch panel by theabove-described naming scheme.

The switching device is equipped with a three-terminal typeconfiguration having a gate terminal that can control a turn-on/offoperation. The three-terminal type switching device is used to detect asignal output from each touch cell. In another embodiment, an additionalswitching device for switching a charging signal applied to each touchcell may be further needed. For example, the three-terminal typeswitching device is a control device for controlling conduction of aninput/output terminal in accordance with a control signal applied to thecontrol terminal of the switching device, and may be any one selectedfrom the group consisting of a relay, a MOS (Metal Oxide Semiconductor)switch, a BJT (Bipolar Junction Transistor) switch, a FET (Field EffectTransistor) switch, a MOSFET (Metal Oxide Semiconductor Field EffectTransistor) switch, an IGBT (Insulated Gate Bipolar Transistor) switch,and a TFT (Thin Film Transistor) switch. The relay is a switching devicethat outputs a voltage or current applied to an input terminal thereofwith no loss when a current is applied to a control terminal thereof.The BJT switch is a switching device that a certain amount of amplifiedcurrent flows from the collector thereof to the emitter thereof whencurrent flows to the base thereof at a state where a voltage higher thana threshold voltage of the base thereof is applied to the base. Inaddition, the TFT switch is a switching device that is used in a pixelunit constituting a display device such as LCD or AMOLED, including agate terminal as a control port, a drain terminal as an input port, anda source terminal as an output port, in which the TFT switch isconducted when a voltage above a threshold voltage larger than thevoltage applied to the source terminal is applied to the gate terminaland a current dependent on a magnitude of the voltage applied to thegate terminal flows from the input port to the output port.

On the following description, an example of using the TFT as a switchingdevice will be described, in which identical reference numerals aregiven to the switching device and the TFT. Switching a signal in eachtouch cell using the TFT is similar to constituting a pixel for a screendisplay using the TFT in LCD, AMLCD (Active Matrix LCD) or AMOLED. Inother words, the touch cells 50 that are described in the presentinvention detect touch inputs in an active matrix type. The technicaladvantages of the touch panel include excellent mass-production and goodreliability, and prevention of a backflow of a signal to thus preventtouch inputs from being misrecognized and to enable recognition of amulti-touch input that is touched at a number of touch pointssimultaneously.

FIG. 2 is an exploded perspective view showing structure of a touchpanel according to the present invention. As shown, a touch panel of asingle substrate 30 is provided on the upper surface of a display device20. The substrate 30 is made of a light transmission material such asglass or film. As shown, drive ICs (Integrated Circuits) 71 for applyinga position detection signal and a gate signal to respective signal linesto be described later are mounted in edge portions of the substrate 30.The illustrated embodiment has been described with respect to a casethat the drive ICs (Integrated Circuits) 71 are implemented as a singleIC as an example, but the drive ICs (Integrated Circuits) 71 may beconfigured separately into ICs for sending and receiving or a gate ICmay be also configured separately.

The drive ICs 71 are mounted at the edge portion of the substrate 30 inthe form of a Chip On Film (COF) or a Chip On Glass (COG). In addition,the drive ICs 71 may be configured into an Amorphous Silicon Gate (ASG)in order to reduce a wiring area at the edge portions of the substrate30. The ASG is a System On Glass (SOG) technology that implements a gateIC function on an amorphous silicon glass substrate in which the gate ICfunction can be implemented directly by the ASG on the glass substrate,and a gate installation area of the gate IC can be minimized. Inaddition, the drive ICs 71 can transmit a signal from the outside of thesubstrate 30 by using a Flexible Printed Circuit (FPC).

Meanwhile, a touch panel having the touch cell structure according tothe present invention is made of a single substrate 30, and thus can bemanufactured very slimly. Thus, a touch panel may not be provided on topof the display device 20, as shown in FIG. 2, but can be built in thedisplay device 20. Despite the above-described built-in type design,features such as lightweight, thinness, shortening and compactness ofthe display device 20 are not greatly impeded. This is one of theimportant technological advantages of the present invention. Forexample, in the case of the LCD, a touch panel of a single substrate 30and a polarizing plate are stacked on a liquid crystal panel in which aTFT substrate and a color filter substrate are bonded, and then areinstalled in the inside of a BLU housing. Accordingly, the touch panelcan be embedded within the display device 20. As another example, thesubstrate 30 may be also provided as the same substrate as a colorfilter substrate. For example, touch cells which will be described latermay be formed on the upper or lower surface of a color filter substrate.

Before explaining about a specific embodiment of this invention, aprinciple of detecting a non-contact touch input according to thisinvention will be briefly explained below with reference to FIG. 3. InFIG. 3, it is assumed that when a finger 25 or a conductive touch unitsimilar to the finger approaches to a conductive pad 50, a distancebetween the finger 25 and the conductive pad 50 is an interval “d” and afacing area is “A.” An electrostatic capacitance “C” is formed betweenthe finger 25 and the conductive pad 50 as shown in a right-sideequivalent circuit of FIG. 3 and a numerical formula. If a voltage orcurrent signal is applied to the conductive pad 50 having theelectrostatic capacitance “C,” charges of a magnitude “Q” can beaccumulated and a voltage relationship formula is formed as V=Q/C. Inthis case, the human body is virtually grounded with respect to theearth.

If a predetermined signal is applied to the conductive pad 50 at a statewhere the finger 25 opposes the conductive pad 50 with an interval of“d,” charges are charged in the electrostatic capacitance “C” that isformed between the conductive pad 50 and the finger 25. Here, as shown,since the switching device 40, preferably the gate terminal of the TFTis connected to the conductive pad 50, the TFT 40 is turned on for atime for charging the conductive pad 50 and discharging the signalaccumulated in the electrostatic capacitance “C.” The magnitude of thedischarged signal becomes smaller gradually as time passes, and the TFT40 is turned off if the electrostatic capacitance “C” is discharged to adegree.

The present invention detects non-contact touch inputs by using aphenomenon that the electric potential of the gate terminal of the TFT40 by the electrostatic capacitance “C” that is formed between the touchunit and the conductive pad 50 is varied. Here, since the output signalwith respect to the electric potential of the gate terminal of the TFT40 appears as a logarithmic function as will be described later, theoutput of the TFT 40 has tens of times to tens of thousands of times anoutput difference depending upon whether or not a touch input exists.The present invention employs a P2G type touch cell structure that makesthe electric potential of the conductive pad 50 determine the electricpotential of the gate terminal of the TFT 40, and apparently differsfrom the conventional electrostatic capacitive type touch input deviceand touch cell structure.

FIG. 4 is a diagram showing a touch cell structure in accordance with abasic embodiment of the present invention. In FIG. 4, a touch panelhaving touch cells 60 with a resolution of 3*3 has been illustrated. Thetouch cells 60 are actually arranged in a much higher resolution but,for clarity of the present invention, a touch panel having touch cells60 with a resolution of 3*3 will be illustrated as an example.

Referring to FIG. 4, a plurality of first signal lines 32, second signallines 34 and auxiliary signal lines 37 are arranged on one surface ofthe upper substrate 30. The first signal lines 32 are lines for sendingposition detection signals (or charging signals) to the respective touchcells 60, and the second signal lines 34 are lines for receiving theposition detection signals from the respective touch cells 60. Theauxiliary signal lines 37 are lines for applying auxiliary signals forobserving to the respective touch cells 60. In the illustratedembodiment, the first signal lines 32 and the second signal lines 34 arearranged in parallel with each other, and the auxiliary signal lines 37are arranged crossing two signal lines 32 and 34. However, these signallines have been illustrated only to help comprehension of thisinvention, but all the signal lines 32, 34 and 37 may be wired inparallel with each other or may be wired at a different wiring angle. Inaddition, each signal line can be wired in the form of an obliquepattern or a zigzag pattern.

In the embodiment of FIG. 4, each unit cell includes a conductive pad 50and a three-terminal type switching device 40 whose gate terminal isconnected to the conductive pad 50. The three-terminal type switchingdevice 40 may be implemented into the aforementioned various switchingdevices, preferably a TFT 40. The TFT has been already validated in thefield of Active Matrix LCD (AMLCD), or AMLCD.

As shown, the conductive pad 50 is connected to the first signal line32, and receives charging signal from the first signal line 32. The gateterminal of the TFT 40 is connected to the conductive pad 50, the drainterminal as an input terminal of the TFT 40 is connected to an auxiliarysignal line 37, and the source terminal as an output terminal of the TFT40 is connected to the second signal line 34.

The conductive pad 50 is formed of Indium Tin Oxide (ITO), Carbon NanoTube (CNT), Antimony Tin Oxide (ATO), Indium Zinc Oxide (IZO), or atransparent conductive material having a conductive characteristicsimilar to that of the ITO, CNT, ATO, or IZO. The conductive pad 50forms an electrostatic capacitance as it faces the bodily finger 25. Thearea of the conductive pad 50 functions as an important factor thatdetermines an electrostatic capacitance that is generated at the time ofa touch input. For example, as the area of the conductive pad 50 becomeslarge within the touch cell 60, the electrostatic capacitance that isgenerated at the time of a touch input becomes large.

A system configuration of the touch panel is illustrated at the lowerportion of FIG. 4. As shown, a touch position detector 70 is provided ata one-side edge portion of the panel or the outside of the panel. Thetouch position detector 70 includes a drive IC 71, a timing controller72, a signal processor 73, and a memory unit 74. The detection signalobtained from the touch position detector 70 is transferred to a CPU 75.The CPU 75 may be a CPU for the display device 20, main CPU of acomputer device, or a CPU for the touch input device itself. Although itis not shown in the drawing, the system configuration further includes apower supply for generating a high or low voltage signal for the touchinput detection.

The timing controller 72 generates a time-division signal of tens ofmilliseconds (ms) or less. The signal processor 73 applies a chargingsignal to each first signal line 32 via the drive IC 71. An observingauxiliary signal is applied to each auxiliary signal line 37. The signalreceived from the second signal line 34 is detected to thus acquirecoordinates of the touch cell 60 where a touch input occurs.

The memory unit 74 is a unit of temporarily storing the acquiredcoordinate values. The illustrated embodiment shows a case that thetouch cell 60 has a resolution of 3*3. However, since the touch cell 60has a much higher resolution actually, signals may be lost duringprocessing of many signals. For example, when the signal processor 73 isin a “busy” state, it may not recognize the position detection signal tothus miss the signal. The memory unit 74 prevents the loss of signals asdescribed above.

FIG. 5 is a block diagram conceptually showing an embodiment of thememory unit. Referring to FIG. 5, the memory unit 74 has absoluteaddresses corresponding to the coordinates of the touch cell 60. To thisend, the memory unit 74 has the number of bits greater than the numberof the touch cells 60. If a touch input occurs at the right-lowerportion in the touch cell of the embodiment of FIG. 4, the signalprocessor 73 stores the obtained coordinates in an “m9” address of thememory unit 74 as shown in a dotted line in FIG. 5, and reads the memoryunit 74 after having scanned the whole signals once, to thus determinewhether or not any missing signal or signals exist. If a signalcorresponding to the coordinate in the m9 address has been missed, buthas been stored in the m9 address of the memory unit 74, thecorresponding signal is generated as a normal input signal and erasesthe memory unit 74 prior to a next scanning operation.

FIG. 6 is a plan view showing a configuration of touch cells accordingto a preferred embodiment of the present invention, which shows anexample that two switching units 42 and 44 are included in a touch cell60. The embodiment of FIG. 6 shows an example that the signal processingbecomes easier and the multi-touch input is recognized stably.

As shown in FIG. 6, a plurality of gate signal lines 36 are furtherprovided on one surface of the substrate 30. A basic circuitconfiguration that each touch cell 60 includes a conductive pad 50 and aswitching unit 44 whose gate terminal is connected to the conductive pad50 is the same as that of the embodiment of FIG. 4, but a switching unit42 for supplying the conductive pad 50 with a charging signal is furtherprovided for the basic circuit configuration. The latter switching unit42 is a first switching unit 42 and the former switching unit 44 is asecond switching unit 44. Preferably, both the two switching units 42and 44 are TFTs, respectively.

Referring to FIG. 6, the input terminal of the first TFT 42 is connectedto a first signal line 32, the output terminal thereof is connected tothe conductive pad 50, and the gate terminal thereof is connected to agate signal line 36. The gate terminal of the second TFT 44 is connectedto the conductive pad 50 and the input and output terminals thereof areconnected to an auxiliary signal line 37 and a second signal line 34.

In the embodiment of FIG. 6, a touch position detector 70 sequentiallyapplies a scan pulse to each gate signal line 36, so sequentially turnson the first TFT 42, or turns on a gate signal at the same time, to thusinduce the electrostatic capacitance between the finger 25 and theconductive pad 50 to be charged and then applies a scan poise to theauxiliary signal line 37 to determine the touch position.

FIG. 7 is a plan view showing structure of a unit touch cell structurein the embodiment of FIG. 6, and FIG. 8 is a cross-sectional view cutalong a line I-II of FIG. 7. The structure of the unit touch cell 60will be described in more detail with reference to FIGS. 7 and 8 asfollows. Referring to FIG. 7, a first TFT 42 and a second TFT 44 areconnected to a conductive pad 50 and signal lines as shown in thecircuit diagram of FIG. 6. As an embodiment, the signal lines arepreferably formed of aluminum series metal such as aluminum and aluminumalloys, silver series metal such as silver and silver alloys, copperseries metal such as copper and copper alloys, molybdenum series metalsuch as molybdenum and molybdenum alloys, chrome, titanium, andtantalum. A first signal line 32, a second signal line 34, a gate signalline 36 and an auxiliary signal line 37 may include two films havingrespectively different physical properties, that is, a lower film (notshown) and an upper film (not shown) on the lower film. The upper filmis made of metal of a low specific resistivity, for example, aluminumseries metal such as aluminum and aluminum alloys, so as to reducesignal delay or voltage drop. In contrast, the lower film is made of amaterial having an excellent contact feature with respect to Indium TinOxide (ITO) and Indium Zinc Oxide (IZO), for example, molybdenum (Mo),molybdenum alloys, chromium (Cr), etc.

The signal lines are preferably formed of a transparent conductivematerial, and thus are prevented from being seen by observers. Althoughit is not shown in the drawings, when the signal lines are formed of thetransparent conductive material, metal series signal lines may be usedin part in order to insulate between the signal lines at theintersection of the signal lines and reduce resistance of the signallines. In addition, although it is not shown in the drawings, the signallines may be protected with insulation films. If the signal lines aremade of the transparent material, the signal lines may not be onlyprevented from being seen, but a moire phenomenon due to an opticalinterference with a black matrix (BM) that is formed between the signallines (for example, such as a gate line and a source line of LCD) orpixels for screen display of the display device to thus conceal thesignal lines may be also prevented. The signal lines that are formed indifferent kinds of layers are connected with other components viacontact holes 59.

Referring to FIG. 8, a gate insulation film 43 made of silicon nitride(SiNx) is formed on the gate terminal 56 of the first TFT 42 and thesecond TFT 44, respectively. An active layer 46 is formed on top of thegate insulation film 43 in which the active layer 46 overlaps the gateterminal 56 and forms a channel between the drain terminal 57 and thesource terminal 58. In addition, the active layer 46 is also formed tooverlap the drain terminal 57 and the source terminal 58. The activelayer 46 is formed of hydrogenated amorphous silicon or polycrystallinesilicon. An ohmic contact layer 47 made of a material of n+ hydrogenatedamorphous silicon in which high concentrations of silicide or n-typeimpurities are doped is formed on the active layer 46. The ohmic contactlayer 47 is a layer for ohmic contact between the drain terminal 57 andthe source terminal 58. A protective film 45 is formed on the drainterminal 57 and the source terminal 58, respectively. A conductive pad50 that is formed of a transparent conductive material such as ITO islocated on the upper surface of the protective film 45.

As shown, in order to connect the conductive pad 50 to the sourceterminal 58 of the first TFT 42 and the gate terminal 56 of the secondTFT 44, the contact holes 59 are used. The contact holes 59 can be madeinto different shapes such as a polygon or circle.

Although it is not shown in the drawings, light shield layers forblocking light can be formed on the TFTs 42 and 44. The material that isused to manufacture the drain terminal 57 and the source terminal 58 ofthe TFTs 42 and 44 or the material that is used to manufacture the gateterminal 56 can be used as the light shield layers. The light shieldlayers prevent the TFTs 42 and 44 from malfunctioning in response tolight.

FIG. 9 is a waveform diagram illustrating an example of detecting touchinputs according to the embodiment of FIG. 6. Referring to FIG. 9, thetouch position detector 70 offers a scan pulse sequentially to each gatesignal line 36. The gate signal Gn offered by the touch positiondetector 70 has a voltage level of a sufficient size so that the gateterminal of the first TFT 42 enters an active area. For example, thegate signal Gn is preferably set to be larger by 3V or higher than theposition detection signal Dn that is transmitted via the first signalline 32. In the case of a preferred embodiment of the present invention,the high (Hi) voltage level of the position detection signal Dn is 13Vand the high (Hi) voltage level of the gate signal Gn is 18V. Inaddition, in order to turn off the first TFT 42 stably, the gate OFFvoltage is set to be at a range of −5 to −7V.

The gate signal Gn has enough observation time between the respectivesignals. This is to make the virtual capacitor formed between the fingerof the human body and the conductive pad 50 according to an approach ofthe human body, have a sufficient charging time. As illustrated, a pauseperiod of a sufficient observation time 1 is given between the gatesignals G1 and G2. In the case that any one of the gate signals Gn is ata high state (Hi), the position detection signal Dn that is appliedthrough the first signal line 32 is offered to keep a high state (Hi)necessarily. Preferably, when one gate signal Gn is at a pause period,the position detection signal Dn also has a slight pause period.

The touch position detector 70 offers an observation voltage through theauxiliary signal lines 37. The auxiliary signal Auxn that is appliedthrough the auxiliary signal line 37 should be necessarily at a highlevel at a part of an interval of the observation time, but may bealways offered at a high level at the whole interval of the observationtime. The auxiliary signal Auxn offers an observation voltage lower by3V or more than 13V that is a voltage that is charged between the finger25 and the conductive pad 50 by the position detection signal Dn at ahigh (Hi) level. For example, it is enough that the observation voltageof the auxiliary signal Auxn is about 5V.

Referring to FIG. 9, the waveform that is obtained through the secondsignal lines 34 and the process of acquiring the touch signal throughthe waveform will follow.

If a human body does not approach although the gate signal has beenapplied and then the observation time has passed, as in the case thatthe gate signals G1 and G2 are applied, the signals Sn that are obtainedthrough the second signal lines 34 have the waveform illustrated. Thisis because the electrostatic capacitance is not formed in the conductivepad 50 since the human body has not approached. In more detail, when thegate signal G1 is applied, the first TFT 42 is turned on. In this case,since the position detection signal Dn is applied to the gate terminalof the second TFT 44, the second TFT 44 is also turned on. By the way,because wiring resistance and parasitic electrostatic capacitance of thesecond signal line 44 exist, the signals S1 and S2 that are received asillustrated have a curved line in a section ascending up to a high (Hi)level and a section descending down to a low (Lo) level, respectively.As illustrated, it is assumed that a time taken from the immediate timeafter the first TFT 42 has become turned off by the gate signal G1 andhas been changed into an observation time, to the time the gate voltageof the second TFT 44 descends sharply and the signal Sn that is obtaineddescends at a low (Lo) level perfectly, is “T1.” Here, a time delay thatoccurs in the output signals Sn in comparison with the input signals Dnin the waveform diagram of FIG. 9 has been ignored.

If a bodily approach is achieved to a right-lower touch cell 60 of FIG.6 at a certain point in time, an electrostatic capacitance will beformed between the conductive pad 50 and the bodily finger 25 in thecorresponding touch cell 60. As can be seen from the waveform of FIG. 9,if a touch occurs in a section where the gate signal G3 is at a high(Hi) level, a virtual capacitor is formed at a moment the bodily finger25 approaches to the conductive pad 50. Here, as the waveform of S3 isdistorted at a touch occurrence point in time in the waveform diagram ofFIG. 9, charge voltage can be varied at a charge beginning time.However, the S3 waveform rises up to a high (Hi) level as soon ascharging is ended.

By the way, in the case that the mode of the G3 signal is changed intoan observation time, that is, in the case that the G3 signal is turnedoff, voltage that is charged in the virtual capacitor is discharged, andthe gate voltage of the second TFT 44 descends slowly. As can be seenfrom the S3 waveform, the output waveform of the second TFT 44 exhibitsa unique output characteristic. Here, a time that is taken for the Snwaveform to fall down to 50% or below is assumed as “T2.”

Referring to the waveform diagram of FIG. 9, it can be seen that thetime points T1 and T2 have a considerable time difference therebetween.The touch position detector 70 can acquire a touch signal by reading atime that is taken for the waveform of the signal Sn that has beenobtained through the second signal line 34 after the gate signal Gn hasbeen turned off as described above to descend or descending voltage (orcurrent) at a certain point in time.

The embodiment of FIG. 9 is one embodiment for acquiring a touch signal.It is possible to acquire the touch signal by an alternative method thatdiffers from the FIG. 9 embodiment. For example, according to thealternative method, after all the gate signals Gn have been turned onall at once, to thus induce the virtual capacitor formed between thehuman body and the conductive pad 50 to be charged, signals aresequentially applied to the auxiliary signal lines 37 to thereby observeoutput waveform. It is obvious to one of ordinary skill in the art thatthe method of acquiring the touch signal can be implemented in variousforms according to the technological spirit of the present invention.

FIGS. 10 and 11 are configurational diagrams illustrating otherembodiments of the present invention, respectively, which show anexample that an additional auxiliary capacitor 54 is added between theconductive pad 50 and the ground in each touch cell 60. The addedauxiliary capacitor 54 shares charges with the virtual capacitor that isformed by the finger 25 of the body. Accordingly, the gate potential ofthe second TFT 44 will be dropped or the charging time will becomelonger. Therefore, a touch signal can be acquired more stably withrespect to the approach of the finger 25 by detecting the gate potentialof the second TFT 44 or the charging time.

Referring to FIG. 10, the auxiliary capacitor 54 is further connectedbetween the conductive pad 50 and the auxiliary signal line 37, inaddition to the embodiment of FIG. 6. Even in this embodiment, the touchposition detector 70 may offer a scan pulse sequentially to each gatesignal line 36, or may apply an identical gate signal to all the gatesignal lines 36.

In this embodiment, the gate signal Gn and the auxiliary signal forobservation Auxn do not necessarily need to interoperate, but can beapplied independently. However, if too much time has elapsed after theauxiliary capacitor 54 has been charged by the gate signal Gn, theauxiliary capacitor 54 may be freely discharged to thus cause failure ofobservation. Accordingly, it is desirable that the auxiliary signal Auxnshould be applied immediately after the auxiliary capacitor 54 has beencharged by the gate signal Gn.

As an embodiment, the turn-on (ON) voltage of the gate signal Gn is setto be 15V. When the gate signal Gn is applied, even the positiondetection signal Dn is also applied, and the auxiliary capacitor 54connected to the gate terminal of the second TFT 44 is charged. TheHi-level potential of the position detection signal Dn is a voltage thatturns on the second TFT 44, and thus is appropriately about 10Vconsidering the relationship with respect to the gate signal Gn. Theposition detection signal Dn is offered for a sufficient time so as tocharge the auxiliary capacitor 54.

Since the gate voltage of the second TFT 44 is greater by 3V or morethan the voltage of the input terminal Auxn, the second TFT 44 is alwaysturned on. If an approach of the finder 25 is achieved at theright-lower portion of the touch cell 60 at a point in time when theauxiliary signal Auxn for observation is applied, the charges stored inthe auxiliary capacitor 54 are discharged to then move to the virtualcapacitor formed by the human body. This will continue until the twocapacitors are in the same potential. If the electrostatic capacitanceof the auxiliary capacitor 54 is sufficiently smaller than that of thevirtual capacitor formed by the finger 25, the charge sharing occurs anda point in time when the second TFT 44 is not turned on or the size ofthe output signal Sn is degraded takes place in the size relationshipbetween the voltage applied to the gate terminal of the second TFT 44and the auxiliary voltage Auxn, to then be read and to thus acquire atouch signal. In this example, the acquired touch signal has acoordinate value corresponding to “D3, S3.”

Referring to FIG. 11, an auxiliary signal line 37 is provided with afirst auxiliary signal line 37 a and a second auxiliary signal line 37b. In addition, one end of the auxiliary capacitor 54 is connected tothe first auxiliary signal line 37 a and the input terminal of thesecond TFT 44 is connected to the second auxiliary signal line 37 b. Thepresent embodiment of FIG. 11 differs from the embodiment of FIG. 10,only in a point of view that an auxiliary signal for observation and anauxiliary signal for state monitoring are used instead of a singleauxiliary signal, but the former is the same as the latter in the restpoints of view. An auxillary signal Aux1-n is applied to the firstauxiliary signal line 37 a for observation and an auxiliary signalAux2-n is applied to second auxiliary signal line 37 b for statemonitoring.

In this embodiment, the turn-on (ON) voltage of the gate signal Gn isset to be 18V. The high level potential of the position detection signalDn is a voltage that turns on the second TFT 44, and thus isappropriately about 12V. As an example, the auxiliary signal forobservation Auxin may have a potential of −18V at a low level, and 0V ata high level. For example, when the auxiliary signal Aux1-n is at a lowlevel and the auxiliary capacitor 54 has been charged, the gatepotential of the second TFT 44 drops down to −6V. Accordingly, thesecond TFT 44 is not turned on with respect to the second auxiliarysignal line 37 b having a potential which is larger than the gatepotential of the second TFT 44. In addition, since the high levelpotential of the position detection signal Dn is 12V at a high level(that is, zero volt) of the auxillary signal Aux1-n, it is ensured thatthe second TFT 44 is stably turned on with respect to the auxiliarysignal Aux2-n which is less by 3V than the high level potential of theposition detection signal Dn. The auxiliary signal Aux2-n is preferablysynchronized with the auxiliary signal Aux1-n. The potentials at a highlevel and a low level of the auxiliary signal Aux2-n are preferably thesame as those of the auxiliary signal Aux1-n.

In the FIGS. 10 and 11 embodiments, the electrostatic capacitance of theauxiliary capacitor 54 may be selected in various forms, and the voltageapplied to the gate terminal of the second TFT 44 may be adjusted afterhaving performed the charge sharing, which becomes a factor thatdetermines a descending slope of the waveform of the Sn signal when atouch operation occurs. In other words, by adding the auxiliarycapacitor 54, the width of selecting the voltage level of each signal iswidened and the descending slope of the Sn signal is made to become moregradual, to thus obtain a touch signal stably.

The above-described embodiments show touch cell structures in accordancewith the present invention. Each touch cell 60 basically consists of apad to gate (P2G) mode, in which some components can be added to thisbasic configuration. In addition to the above-described embodiments,each touch cell 60 may further includes additional switching devices,capacitors, resistors, or other electrical devices. Here, a very highdetection sensitivity and accuracy and an ability of detecting touchinputs at a digital mode may be listed as the technical characteristicsof each touch cell 60 having the above-described pad to gate (P2G) modesince a kick back at the gate terminal of the switching device 40greatly varies depending on whether or not a touch input exists, and theoutput signal of the switching devices 40 has tens of times or tens ofthousands of times a difference depending upon a kick back differencedue to the touch input.

An example of detecting touch inputs using a kick back in the touch cellstructure according to the present invention will be now describedbelow. The following symbols C1 and C2 denote names and capacities ofcapacitors, respectively. For example, the symbol “C1” denotes acapacitor named as C1, and at the same time denotes a capacitance of C1in size.

FIG. 12 is a cross-sectional view showing an example of capacitorsinternally designed in a TFT. Referring to FIG. 12, capacitors Cgd andCgs are formed between the gate and drain terminals of the TFT andbetween the gate and source terminals thereof, respectively, duringmanufacturing TFTs. As shown, the capacitor Cgd is formed in an areawhere the drain terminal 57 and the gate terminal 56 overlap, and thecapacitor Cgs is formed in an area where the source terminal 58 and thegate terminal 56 overlap. The capacities of the capacitors Cgd and Cgsare determined depending upon width or length of the TFT. For example,the capacitors Cgd and Cgs are designed to 10 fF (femto F) to 300 fF orso depending upon width or length of the TFT. Otherwise, the capacitorsC1 and C2 may be externally mounted but will be described later. In thiscase, the capacitors C1 and C2 are designed to 10 fF to 100 uF.

FIG. 13 shows a configuration of a touch cell illustrating a state wherecapacitors are internally designed in a TFT, FIG. 13 illustrates anexample of a state where a built-in capacitor is added in each of thefirst TFT 42 and the second TFT 44 in the touch cell structure of theFIG. 6 embodiment. As shown, a virtual capacitor Ct is formed betweenthe bodily finger 25 and the conductive pad 50, at the time ofoccurrence of a touch input. A signal output through the output terminalof the first TFT 42 is stored in the virtual capacitor Ct for a certainamount of time. The signal accumulated in the virtual capacitor Ct isdischarged through a discharge path formed by peripheral devices thatare connected to the virtual capacitor Ct.

As illustrated, built-in capacitors C1, C2, and C3 of each TFT functionin a circuit configuration that determines the potential of the gateterminal of the second TFT 44 according to the charging and dischargingoperations of the virtual capacitor Ct. Here, the built-in capacitorsC1, C2, and C3 are about 10 fF to 100 uF, as described above. Thevirtual capacitor Ct may be designed freely by adjusting an interval anda facing area between a touch unit and the conductive pad 50. Forexample, if a large area of the conductive pad 50 is selected, thevirtual capacitor Ct is also designed to have a large capacitance basedon the equation of FIG. 3. Conversely, if a small area of the conductivepad 50 is selected (for example, 1 mm² or less), the virtual capacitorCt is designed to have a small capacitance. Preferably, the virtualcapacitor Ct is selected to have a value equivalent to or a larger valueby several hundred times than those of built-in capacitors C1, C2, andC3. However, in some cases, the virtual capacitor Ct may be selected tohave a smaller value by hundreds of times than those of built-incapacitors C1, C2, and C3. For example, the virtual capacitor Ct may bedesigned to be tens of femto Faraday (fF) or tens of micro Faraday (uF).

FIG. 14 is a waveform diagram illustrating examples of kick backwaveform depending upon whether or not a touch input exists, andillustrates signal waveform in the touch cell structure of FIG. 13.Referring to FIG. 14, a kick back difference depending on whether or nota touch input exists will be described below.

When a turn-on voltage that is applied to the gate terminal of the firstTFT 42 is “VH” and a turn-off voltage thereof is “VL,” a difference involtage according to the turn-on and turn-off of the gate terminal ofthe first TFT 42 becomes a value that is obtained by subtracting theturn-off voltage “VL” from the turn-on voltage “VH.” When the first TFT42 is turned on by applying a voltage of “V1” in magnitude to the inputterminal “In1” of the first TFT 42 and applying a voltage of “VH” inmagnitude to the gate terminal “Cont1” of the first TFT 42, a voltagemeasured at the output terminal “Out1” of the first TFT 42 is a voltageof “V2” as shown as waveform of “Out1-A,” in the case that no touchinput occurs in the conductive pad 50. Here, transient responsecharacteristics due to wiring of signal lines, parasitic resistance,etc., are ignored. When the first TFT 42 is turned off by applying theturn-off voltage “VL” to the gate terminal “Cont1” of the first TFT 42after a predetermined time elapses, a voltage measured at the outputterminal “Out1” of the first TFT 42 drops in voltage. Here, since thebuilt-in capacitors C1, C2, and C3 are connected as shown in the FIG. 13circuit diagram, a kick back voltage “KB1” at the time when no touchinput occurs in the waveform of “Out1-A” is defined as the followingequation (1).

$\begin{matrix}{{{KB}\; 1} = {\left( {{VH} - {VL}} \right)\frac{C\; 1}{\left( {{C\; 1} + {C\; 2} + {C\; 3}} \right)}}} & (1)\end{matrix}$

For example, if VH is 10V, VL is −5V, V1 is 8V, and C1 equals the sum ofC2 and C3, the kick back voltage “KB1” is 7.5V. In other words, V2 dropsfrom 8V to 0.5V in the waveform of “Out1-A.” In addition, this voltagedrop means that the potential of the conductive pad 50 drops from 8V to0.5V.

Meanwhile, in FIG. 14, the waveform marked as “Out1-B” indicateswaveform of a voltage measured at the output terminal “Out1” of thefirst TFT 42, in the case that a touch input occurs according to anapproach of the finger 25 with respect to the conductive pad 50. Otherconditions are the same as the above-described case, but since theelectrostatic capacitance Ct is formed between the finger 25 and theconductive pad 50, a kick back voltage “KB2” at the time when a touchinput occurs in the waveform of “Out1-B” is defined as the followingequation (2).

$\begin{matrix}{{{KB}\; 2} = {\left( {{VH} - {VL}} \right)\frac{C\; 1}{{C\; 1} + {C\; 2} + {C\; 3} + {C\; 4}}}} & (2)\end{matrix}$

If Ct has three times the size of C1 the kick back voltage KB2 is 3V.Namely, V2 drops in the waveform of Out1-B from 8V to 5V.

In the FIG. 14 embodiment, Out2-A and Out2-B illustrate the size of thesignal (the current value in this example) output from the outputterminal Out2 of the second switching unit 44. It can be seen thatOut2-A and Out2-B have similar types of waveforms to those of Out1-A andOut1-B, respectively.

As noted earlier, the size of Ct can be selected by adjusting a gap andthe facing area between the touch unit and the conductive pad 50. Thevalue of the denominator of KB2 becomes large at a high magnification incomparison to KB1 so that Ct can be designed at a high magnification incomparison to C1. Accordingly, a difference between KB1 and KB2 canincrease greatly.

FIG. 15 is a graphical view showing a gate voltage versus output currentcharacteristic of a TFT. Referring to FIG. 15, it can be seen that theoutput signal of the TFT has a logarithmic function in correspondence tothe signal applied to the gate terminal of the TFT. Referring to the 15,when a control voltage Vgs applied to the gate terminal of the TFT is15V, Ids of about 1 uA flows between the drain and source terminals ofthe TFT. Meanwhile, when Vgs is 0V, it can be seen that Ids of 100 pAflows. In other words, when the control voltage voltage-drops from 15Vto 0V, the output current has about ten thousand times the difference.

In other words, a difference between KB1 and KB2 can be properlyselected by appropriately selecting C1, C2, C3, and the magnificationsof Ct considering C1, C2, and C3, and accordingly the output signal ofthe second TFT 44 may make tens of times to tens of thousands of times adifference. Thus, the present invention has technical advantages thattouch inputs can not only be easily detected, but also very highdetection accuracy and reliability are ensured, and the touch inputs canbe detected at a digital mode that detects a high/low level of a signal.In addition, width or breadth of the touch cell 60 can be taken on avery small scale by these technical advantages.

Meanwhile, in the above-described embodiment, a TFT has been referred toas a switching device 40. The built-in capacitors exist in the TFT dueto the structure that gate metal and source metal are laminated as shownin FIG. 12, but in the case that the TFT is substituted with the otherswitching device (where a built-in capacitor does not exist) other thanthe TFT, a kick back effect can be obtained by adding a capacitor to theswitching device 40 as shown in FIG. 13. In addition, in theabove-described embodiment, a voltage driven type TFT has been referredto but the driving method and detection method may vary in the case thatthe TFT is replaced by the other switching device. For example,switching devices such as BJT or IGBT operate at a current-driven mode,and output tens of times or more current in comparison with the currentapplied to the control terminal. Therefore, the switching devices suchas BJT or IGBT are given a kick back difference depending on whether ornot a touch input occurs, respectively, and thus obtain an outputcurrent characteristic tens of times or more in comparison with acontrol current of a small difference.

FIG. 16 is a diagram showing an example of detecting a touch input usinga comparator, and FIG. 17 is a waveform diagram illustrating waveform atthe time of detection using a comparator. FIGS. 16 and 17 show anexample of recognizing a touch input by detecting a high/low level ofthe output signal of the second TFT 44 at a digital mode. As shown inFIG. 16, the signal Sn output from the second TFT 44 is input to thecomparator and is compared with a reference signal. As noted earlier,since a difference of the output signal output from the second TFT 44becomes large depending on whether or not a touch input occurs, thecomparator can obtain a comparison result very easily. In addition, theoutput of the comparator is a digital signal having a high or low level.The touch position detector 70 can read the digital signal withoutadditional signal conversion.

For example, as shown in FIG. 17, when a touch input occurs and theoutput signal Sn of the second TFT 44 becomes high at an interval of t1,Sn gets greater than a reference signal and the output of the comparatorbecomes high or low depending on configuration of the circuit. Sn dropsto a low level, at an interval of t2 where a touch input is interruptedor a signal will be extinguished after a specified time interval. Inthis case, Sn is smaller than the reference signal, and the output ofthe comparator becomes high or low depending on configuration of thecircuit. Thus, the touch position detector 70 can process the output ofthe comparator in a digital mode. Here, although the difference betweenthe high and low states of Sn seems to be small in the illustratedexample, this difference gains tens of times to tens of thousands oftimes, as described above.

In the case that the touch input is detected by using the comparator asshown in the FIGS. 16 and 17 examples, the reference signal is used. Thereference signal can be generated by the touch position detector 70including a separate reference signal generator.

However, a given constant reference signal can cause a reading errorwith respect to the touch input. For example, it is assumed that thecharacteristic of the first TFT 42 varies due to factors such astemperature or aging and thus a voltage fluctuation of 2V or so occursat Vgs for producing the same output current. Referring to the FIG. 15graph, the difference of the output current Ids gains one hundred timesat an interval where Vgs is changed from zero (0) to 2V or so. If it isdetected whether or not a touch input occurs based on a differencebetween when Vgs is zero (0) and when Vgs is 15V, the Ids differencegains ten thousands of times. Accordingly, the reference signal can bedetermined as an intermediate value (that is, a current value of onehundred times a difference) of the output signal of the second TFT 44depending upon whether or not a touch input occurs. That is, thereference signal can be set as a current value at Vgs of 2V. Here, theintermediate value does not necessarily mean a central value. Forexample, a current value with ten times the difference or one thousandtimes the difference may become the reference value.

By the way, when the output voltage of the first TFT 42 changes by 2V,the output of the second TFT 44 approaches the reference signal althoughno touch input occurs. Here, malfunction may occur due to an influenceof disturbances, in which it is misrecognized as if a touch input weredetected although no touch input occurs.

The present invention uses a sensing cell 61 in order to preventmalfunction according to setting of the reference signal. The sensingcell 61 is installed on the panel, and is configured into a similarstructure to the touch cell 60 for detection of the touch input. Thesensing cell 61 having a structure similar to the touch cell 60 has thesame temperature conditions and aging as the touch cell 60. For example,when the TFT of touch cell 60 changes by 2V in voltage due to changes oftemperature and aging, the sensing cell 61 also generates a referencesignal of the same voltage fluctuations as that of the TFT. Therefore,reading errors due to factors such as temperature and aging can bereduced.

FIGS. 18 and 19 show an example of the sensing cell 61, respectively. Asshown in the FIGS. 4 and 6 embodiments, the touch input can be detectedthrough the output signal of the TFT 40 or second TFT 44. In the FIG. 18embodiment, the sensing cell 61 can be configured into a single sensingswitching device 64 in correspondence to the case that the output signalis obtained via the single TFT as described above. In this example, thesensing switching device 64 is a TFT (hereinafter, called as a sensingTFT 64), and is given as the same reference numeral.

In some cases, an additional TFT can be further added at the rear end ofthe TFT 40 or second TFT 44. In this case, as shown in FIG. 19, thesensing cell 61 may be configured to have a first sensing TFT 66 and asecond sensing TFT 68 that are connected in sequence. In the illustratedexample, although an installation location of the sensing cell 61 is notspecified, the sensing cell 61 may be installed in a non-active area onthe panel.

The sensing TFT 64 that constitutes the sensing cell 61 has the samecircuit configuration as that of the TFT 40 or second TFT 44 that isprovided at the rear end of the touch cell 60 of the conductive pad 50.The drain terminal of the sensing TFT 64 is connected to the auxiliarysignal line 36 and the source terminal thereof is connected to thesecond signal line 34. However, the conductive pad 50 is not connectedto the gate terminal of the sensing TFT 64, and a separate gate signalcan be applied to the gate terminal thereof. As another example, a TFThaving the same circuit configuration as that of the first TFT 42 may beadded to the gate terminal thereof.

The touch position detector 70 applies a predetermined control signal tothe gate terminal of the sensing TFT 64. This control signal correspondsto an intermediate value between a signal applied to the controlterminal of the second TFT 44 of the touch cell 60 at the time ofoccurrence of no touch inputs and a signal applied to the controlterminal of the second TFT 44 of the touch cell 60 at the time ofoccurrence of touch inputs. For example, in the above-described example,2V is applied to the control terminal of the sensing TFT 64. If 2V isapplied to the gate terminal of the sensing TFT 64, the output of thesensing TFT 64 will correspond to value of the Ids in the case that Vgsis 2V. However, if the TFT of the touch cell 60 causes voltagefluctuations due to changes of the temperature or aging, the sensing TFT64 also causes voltage fluctuations under the same condition.Accordingly, the reference signal output from the sensing TFT 64 alsochanges. Therefore, malfunctions due to the above temperature conditionsand aging and reliably can be prevented and touch inputs can be stablydetected.

The invention has been described with respect to the preferredembodiments. However, the invention is not limited to the aboveembodiments, and it is possible for one who has an ordinary skill in theart to make various modifications and variations without departing offthe spirit of the invention defined by the claims.

The invention claimed is:
 1. A touch panel comprising: a lighttransmissive substrate; touch cells that are arranged in a matrix formon the light transmissive substrate and located in an active area of thelight transmissive substrate, in which each touch cell comprises: aconductive pad; a first three-terminal type switching device whose gateterminal is connected to a first signal line and whose output terminalis connected to the conductive pad, and configured to be turned on/offaccording to a control signal applied to a gate terminal of the firstthree-terminal type switching device, thereby supplying a chargingsignal to the conductive pad; and a second three-terminal type switchingdevice whose gate terminal is connected to the conductive pad and whoseoutput signal is changed in correspondence to a change in electricpotential of the gate terminal of the second three-terminal typeswitching device by an electrostatic capacitance between a finger of ahuman body or a touch unit having an electrical characteristic similarto the finger and the conductive pad; and a touch position detector thatrecognizes a touch input from the output signal of the secondthree-terminal type switching device, wherein the electric potential ofthe gate terminal of the second three-terminal type switching devicedepends on a kick back difference between at the time when no touchinput occurs and at the time when a touch input occurs, and wherein thefirst signal line is a signal line for applying a charging signal to theconductive pad, the control signal is applied by a second signal lineother than the first signal line, and a multi-touch input is recognizedby sequentially conducting the first three-terminal type switchingdevice with sequentially applied scan pulse.
 2. The touch panelaccording to claim 1, further comprising first and second capacitorsthat are connected between the gate terminal and the output terminal ofthe first three-terminal type switching device and the secondthree-terminal type switching device, respectively.
 3. The touch panelaccording to claim 2, wherein the first capacitor connected between thegate terminal and the output terminal of the first three-terminal typeswitching device is in the range of 10 fF to 100 uF.
 4. The touch panelaccording to claim 2, wherein the first capacitor connected between thegate terminal and the output terminal of the first three-terminal typeswitching device is selected to have a smaller value by twice to severalhundreds of times than a value of an electrostatic capacitor formedbetween the touch unit and the conductive pad.
 5. The touch panelaccording to claim 2, wherein the first capacitor connected between thegate terminal and the output terminal of the first three-terminal typeswitching device is selected to have a value equal to or greater than avalue of an electrostatic capacitor formed between the touch unit andthe conductive pad.
 6. The touch panel according to claim 2, wherein thefirst and second capacitors are built in the first three-terminal typeswitching device and the second three-terminal type switching device,respectively.
 7. The touch panel according to claim 2, wherein the firstand second capacitors are provided in the outside of the firstthree-terminal type switching device and the second three-terminal typeswitching device, respectively.
 8. The touch panel according to claim 2,further comprising a third capacitor connected between an input terminaland the gate terminal of the second three-terminal type switchingdevice.
 9. The touch panel according to claim 1, wherein the first andsecond three-terminal type switching devices are any one selected fromthe group consisting of a relay, a MOS (Metal Oxide Semiconductor)switch, a BJT (Bipolar Junction Transistor) switch, a FET (Field EffectTransistor) , a MOSFET (Metal Oxide Semiconductor Field EffectTransistor) switch, an IGBT (Insulated Gate Bipolar Transistor) switch,and a TFT (Thin Film Transistor) switch.
 10. The touch panel accordingto claim 1, wherein the touch position detector applies an on/offcontrol signal to the gate terminal of the first three-terminal typeswitching device, applies a position detection signal to an inputterminal thereof, and compares a difference of the output signal of thesecond three-terminal type switching device according to a difference ofa kick back depending upon whether or not a touch input exists, to thusrecognize the touch input.
 11. The touch panel according to claim 10,wherein the output signal of the second three-terminal type switchingdevice has tens of times or tens of thousands of times a differencedepending upon whether or not a touch input exists.
 12. The touch panelaccording to claim 10, further comprising a comparator for comparing theoutput signal of the second three-terminal type switching device and areference signal.
 13. The touch panel according to claim 12, furthercomprising a sensing cell, wherein the sensing cell includes athree-terminal sensing switching device, which has the same circuitconfiguration as the second three-terminal type switching device but toa gate terminal of which the conductive pad is not connected, and isprovided at one side of the light transmissive substrate, wherein thetouch position detector applies a control signal corresponding to avalue positioned between a signal applied to the gate terminal of thesecond three-terminal type switching device of the touch cell at thetime of no occurrence of touch inputs and a signal applied to the gateterminal of the second three-terminal type switching device of the touchcell at the time of occurrence of touch inputs, to the gate terminal ofthe three-terminal sensing switching device , the same signal as thesignal applied to an input terminal of the second three-terminal typeswitching device is applied to an input terminal of the three-terminalsensing switching device, and a signal output from an output terminal ofthe three-terminal sensing switching device is used as the referencesignal of the comparator.
 14. The touch panel according to claim 1,wherein the touch position detector further comprises a memory unithaving addresses corresponding to coordinates of the touch cell, inwhich if a touch input is detected, coordinate values of thecorresponding touch cell are stored in the corresponding addresses ofthe memory unit.